The present invention relates to the separation of benzothiophene compounds from a mixture of hydrocarbons comprising them, for example from a fuel, such as gas oil. The importance of such a separation has increased in recent years as a result of the introduction or the envisaged application of various laws across the whole world targeted at lowering the level of sulfur-comprising products in gas oil.
In the description which follows and in the claims, the expressions given hereinbelow have the following respective meanings:
xe2x80x9cbenzothiophene compoundsxe2x80x9d means both benzothiophene and its homologues, for example dibenzothiophene, and the mono-, di- or trisubstituted derivatives of these, for example dialkyl, trialkyl, alkenyl and aryl;
xe2x80x9celectron-withdrawingxe2x80x9d means any organic compound which is deficient or impoverished in electrons and in particular substituted by groups which are themselves electron-withdrawing groups, for example the sulfo, nitro, halo, haloalkyl, for example trifluoromethyl, cyano, carbonyl, carboxyl, amido or carbamido groups or a combination of these;
xe2x80x9cgas oilxe2x80x9d means, for example, a diesel engine fuel, a kerosene, a heating oil and other fuel oils exhibiting a boiling temperature generally of between approximately 175xc2x0 C. to [sic] approximately 400xc2x0 C.
Among the molecules present in such mixtures of hydrocarbons, for example in gas oil, benzothiophene compounds, in particular dialkyldibenzothiophenes, for example 4,6-dimethyldibenzothiophene (DMDBT), are known as being among the most resistant to the usual catalytic processes for deep hydrodesulfurization. For this reason, the present invention will be more particularly described and explained with respect to the separation of this molecule.
The conventional hydrodesulfurization process mentioned above, which is well known per se, requires fairly drastic and expensive operating conditions to remove or only reduce the content of dibenzothiophene derivatives in mixtures of hydrocarbons, for example gas oils, which limits its industrial application. There has consequently been a search to find a process which makes it possible to separate this group of molecules selectively and efficiently from the other constituents of such mixtures.
U.S. Pat. No. 5,454,933 discloses an approach for reducing the content or selective separation of dibenzothiophene (DBT) or its derivatives in a gas oil feed by adsorption of the dibenzothiophene molecules on solid supports, such as activated carbon, zeolites, silica/alumina, and the like. The selection carried out is based essentially on the shape of the molecule to be removed, that is to say that only steric factors come into consideration in the application of this process. This method has proved to be efficient but its practical and economic advantage is limited by two characteristics of the materials used as adsorbents. On the one hand, their adsorption capacity for dibenzothiophene does not exceed 12% by weight and, on the other hand, their selectivity with regard to aromatic compounds, such as 1-methylnaphthalene (MN), measured by the separation factor xcex1DBT/MN=[DBT/MN]ads/[DBT/MN]soln) [sic], is only 7 in the best of the cases cited.
The present invention has been set the problem of at least partially, indeed even completely, in an inexpensive way, separating the benzothiophene compounds as defined above from a mixture of hydrocarbons comprising them, for example gas oil, by the application of another treatment route which is independent of the only steric factors touched on above in the above United States patent.
This problem has been solved, surprisingly, by the application not of a principle of shape selectivity as described in the abovementioned patent but by the application of the principle of interaction of donor-acceptor type (or of charge transfer type) in carrying out the separation of the benzothiophene compounds.
A subject matter of the present invention is consequently a process for the separation of at least one benzothiophene compound from a mixture of hydrocarbons comprising it, the process being more particularly characterized in that said mixture or a fraction obtained from the latter is brought into contact with a reagent comprising a xcfx80-acceptor complexing agent, in order to obtain a donor-acceptor complex between the acceptor complexing agent and said benzothiophene compound, and in that said complex is separated from said mixture or from said fraction, in order to obtain a fraction impoverished in or purified from said benzothiophene compound.
The application of the principle given above makes possible a very selective separation (xcex1 greater than 100) with respect to non-sulfur-comprising aromatic compounds and a much more efficient separation (capacity, for example, of the order of 30% to 50%) than according to the process disclosed in the abovementioned United States patent.
A process according to the present invention makes it possible to obtain a desulfurized mixture of hydrocarbons exhibiting a sulfur content of between 0 ppm and 2000 ppm, preferably between 0 ppm and 500 ppm.
A process according to the present invention makes it possible to decrease the load of benzothiophene compounds to a value of between 0% and 75% and preferably between 0% and 15%, with respect to the initial weight of said compounds.
In a preferred form of the invention, the process is carried out in a homogeneous phase, that is to say without the intervention of a solid phase for attaching or supporting the xcfx80-acceptor complexing agent.
In another preferred form of the invention, the process is carried out in a heterogeneous phase, that is to say with the intervention of a solid phase as defined above.
The process for the separation of benzothiophene compounds can also be carried out before or after the stage of deep catalytic hydrodesulfurization known per se. Advantageously, and in order to render the separation process more advantageous from an economic view point, the latter is carried out before a stage of deep catalytic hydrodesulfurization, in which case the fraction impoverished in benzothiophene compound is subjected to a deep catalytic hydrodesulfurization. This is because the process according to the invention makes it possible in particular to remove the dibenzothiophene compounds and thus makes it possible to carry out the stage of deep catalytic hydrodesulfurization under milder temperature and pressure conditions and thus to prolong the lifetime of the catalyst.
The complex is preferably separated from the mixture by extraction with an organic solvent, for example chloroform.
Furthermore, it is preferable to regenerate the reagent by separating the complex into benzothiophene compounds and complexing agent. The separation of the complex is preferably carried out chemically but can also be carried out by application of physicochemical means.
The reagent can be regenerated:
by reducing the complex separated from the mixture, in order to form a salt of the xcfx80-acceptor agent;
and by reoxidizing the salt, in order to regenerate the xcfx80-acceptor complexing agent.
In accordance with the present invention, the xcfx80-acceptor complexing agent comprises an electron-withdrawing compound or a compound which is impoverished in electrons. This agent is said to be a xe2x80x9cxcfx80-acceptorxe2x80x9d agent because, generally, it possesses a system of xcfx80 electrons or of xcfx80 type. The xcfx80-acceptor complexing agent preferably comprises an aromatic compound substituted by at least one electron-withdrawing group chosen more preferably from the group consisting of the sulfo, nitro, fluoro, trifluoromethyl, cyano, carbonyl, carboxyl, amido and carbamido groups. A preferred example of such a xcfx80-acceptor complexing agent is chosen from a group consisting of the family of substituted or unsubstituted quinones, more preferably dichlorodicyanobenzoquinone, anthraquinone, benzoquinone or tetracyanoquinodimethane, or the family of substituted or unsubstituted fluorenones, more preferably tetranitrofluorenone or dinitrofluorenone. Among these compounds, tetranitrofluorenone and tetracyanodiquinodimethane [sic] are even more preferred in constituting the xcfx80-acceptor complexing agent because of their increased separation factors and capacities.
According to a preferred embodiment of the invention, the reagent comprises a support, to which the xcfx80-acceptor complexing agent is attached, which is in the divided or undivided form and which is chosen from the group consisting of inorganic oxides, such as alumina and silica, activated charcoal, ion-exchange resins and zeolites. xe2x80x9cIn the divided formxe2x80x9d means that the support can be in particular in the form of beads, for example glass beads, or of granules. The xcfx80-acceptor complexing agent can be supported and on the reagent [sic] by any appropriate means, for example by adsorption, absorption or covalent bonding.
According to another preferred embodiment of the invention, the reagent is composed essentially of the xcfx80-acceptor complexing agent and, in this case, it is more preferably a polymer of the latter. Polymer means both homopolymers formed solely from monomers of the xcfx80-acceptor complexing agent and copolymers of the latter with other polymers [sic].
Depending on the desired degree of separation of the benzothiophene compounds, the mixture or fraction of hydrocarbons to be treated and the applicational flow rate of the mixture or fraction, the time during which the mixture of hydrocarbons is in contact with the complexing agent can vary quite widely. Advantageously, the period during which the contacting operation is carried out can be between approximately 10 minutes and 150 hours and preferably between 2.5 hours to [sic] 115 hours.
The mixture of hydrocarbons comprising the benzothiophene compounds can be brought into contact with the reagent continuously, systematically or nonsystematically, for example in a column.
In such cases, the applicational flow rate of the mixture or of the fraction comprising the benzothiophene compounds to be separated can be between 0.5 ml/min to [sic] 50 ml/min per cm3 of column volume and preferably between 0.5 ml/min to [sic] 10 ml/min per cm3 of column volume.
Furthermore, the temperature at which the contacting operation is carried out can be between 10xc2x0 C. and 60xc2x0 C. and preferably between 15xc2x0 C. and 30xc2x0 C.
The separation process according to the present invention can be carried out batchwise, semi-continuously or continuously. However, in particular for reasons of economy, it is preferably carried out continuously.
The mixture of hydrocarbons is advantageously brought into contact with the reagent with stirring.
The mixture of hydrocarbons is preferably a gas oil, as defined above, and more preferably a diesel engine fuel.